1. Field of the Invention
This invention relates to apparatus for detecting the location of an axially movable, magnetically permeable, elongated member using electrical coils spaced along the axis of movement of the elongated member. More particularly, it is directed to such an apparatus which generates a digital signal representative of the location of the end of the elongated member with switching between discrete values of the digital signal precisely controlled relative to the location of the coils.
2. Prior Art
There are many applications where it is desirable to known the precise location of an axially movable metallic rod. One such application is the monitoring of the position of the control rods in a nuclear reactor. As used herein, the term "control rod" is meant to encompass any of the various rods which are inserted into and retracted from the reactor core whether they are used for normal control purposes in the precise sense in which that term is used in the nuclear field or for other purposes.
For pressurized water reactors, it is of the utmost importance to know the accurate position of each of the control rods. Difference in penetration of over 15 inches between neighboring rods are considered unsafe. In addition, accurate knowledge of the position of the control rods versus thermal power output can be used to determine the condition of the reactor fuel. Furthermore, for certain rods, such as the water displacer rods in an advanced pressurized water reactor, the location of the rods must be known within inches in order to assure proper alignment for latching mechanisms. Hence, for these reasons and others, it is important to have an extremely reliable rod position indication system with suitable accuracy in order to maintain a safe and reliable operating condition of the reactor.
The control rods in a pressurized water reactor move within a pressure vessel and are attached to drive rods which can be incrementally moved in a forward or reverse direction by a drive mechanism such as the magnetic jack mechanism described in U.S. Pat. No. 3,158,766. The drive rods extend longitudinally through the pressure vessel along the axis of movement of the control rods into the sealed, pressurized environment of the rod travel housings. Since it is of the utmost importance to maintain the sealed integrity of the pressurized vessel, mechanical penetrations are kept to a minimum to reduce the likelihood of loss of the pressurized environment. Accordingly, no mechanical penetrations are permitted for detecting the relative position of the control rods within the core of the reactor. Inasmuch as it would be very difficult to detect the position of the actual control rods, it has been the practice to detect the position of the drive rods and to derive control rod position therefrom.
There have been a number of schemes devised for detecting drive rod position. Early systems used the change in the impedance of a coil effected by the end of the drive rod as it moved through the coil to generate an analog signal indicative of rod position. The susceptability of such analog systems to variations in temperature, rod magnetization, permeability of the rod, and power supply voltage and frequency, and to interference from adjacent control rods and drive mechanisms, led to the development of digital position indicating systems such as that disclosed in U.S. Pat. No. 3,846,771. This system uses a number of discrete coils spaced along the linear path traced by the end of the rod. The change in the impedance of each coil in sequence as the rod advances through successive coils in used to generate discrete output signals or one output signal with discrete levels. In one embodiment, adjacent coils are each connected to differential amplifiers so that the largest differential signal, which is generated by the amplifier connected to one coil which has been penetrated by the end of the rod and one which has not, indicates rod position.
U.S. Pat. No. 3,858,191 utilizes two sets of these discrete coil detectors interleaved to provide redundancy. In this system, the differential a-c signals in each set of coils are each d-c restored and passed through a low pass filter to extract the d-c component. These d-c difference signals are then applied to the non-inverting inputs of comparators in a tracking level detector/encoder circuit in which the outputs of the comparators are fed back to the inverting inputs on a common bus so that only the comparator to which the difference signal of largest magnitude is applied is switched to the active state. The comparator outputs also generate a coded digital signal representative of the largest difference signal in the set which is processed with the coded signal from the other set to generate a display illustrating the position of the control rod. U.S. Pat. No. 3,893,090 discloses a similar interleaved, redundant system in which the processing means adjusts the display of rod position to the coarser resolution out of service.
While the above digital rod position indicating systems are reliable and provide suitable resolution of rod position, the accuracy of the points relative to the coils at which the transition from one coded signal to the next occurs is dependent on many factors. The most evident of these are: the tolerances of the coils, the tolerances of the resistance components, leakage currents of the capacitive components, hysteresis of the comparators and variations in the magnetic flux density over the range of rod travel. Tight control of these factors such as through careful matching of the coils adds to the cost of the system, and may not even then provide the accuracy required by the user. In such an instance, individual fine adjustment of each transition may be necessary, but becomes cumbersome when high accuracy is required at a large number of points.